Planning synthesizers and the methodologies used to automatically develop plans are typically considered to be in that domain of computer science known as "artificial intelligence".
Most prior art automated plan synthesizers develop plans that are procedural in nature. This means that such programs assume that if the internal state of the plan execution system is known, then there is a single action (sometimes called a "possible future") that will best advance the system toward its goal. Such plan synthesizing systems generally do not allow for unexpected events in the external world, and thus are not "reactive".